Abstract
Dual-fuel diesel/natural gas direct-injection engine is promising and highly attractive due to its low-carbon emission and high thermal efficiency, and both high-pressure diesel and natural gas injections are critical for air–fuel mixing. This study presents an optical experimental investigation on the high-pressure dual-fuel diesel/methane injection process based on a constant-volume vessel test rig. The results show that the diesel penetration process of the dual-fuel injection experiences two stages: Stage I, the diesel tip penetration Sdiesel, the diesel spray area Adiesel, and the diesel spray perimeter Cdiesel of the dual-fuel injection are smaller than those of the single diesel injection. Stage II, both the diesel and methane continue to penetrate forward, and Sdiesel, Adiesel, and Cdiesel of the dual-fuel injection become larger than those of the single diesel injection do. The diesel injection pressure causes effect on the dual-fuel spray penetration. The diesel injection pressure directly causes linear influence on the two-stage dual-fuel injection characteristic. As the diesel injection pressure increases, the diesel spray meets the methane jet advancer and the cross point occurs linearly earlier. Furthermore, the dual-fuel injection is asymmetric and the methane gas jet enhances this asymmetry so that the spray cone shifts to the side of the methane gas jet.
1. Introduction
Natural gas (NG) is a promising gas fuel and a good alternative energy source for traditional liquid fossil fuels of modern power devices. NG mainly contains methane, and this low-carbon gas fuel has been widely applied in many power devices such as the internal combustion engines for vehicles, ships, and power plants.1,2 Most modern internal combustion engines powered by gas fuels utilize pulsed fuel injection system to realize air–fuel mixture formation, and gas fuel direct injection (DI) is helpful for improving efficiency. Natural gas fuel as one alternative fuel is a promising focus because of its abundant storage, widespread distribution infrastructure, and low cost together with clean-burning qualities,3,4 and the direct-injection compression ignition (DICI) technology benefits the NG engines with high thermal efficiency and low emission.5,6 The direct-injection technology requires high pressure of the gas fuel, and high-pressure gas fuel direct injection is necessary for higher efficiency and output power. McTaggart et al.7 tested a direct-injection natural gas engine and found that both the engine efficiency and output power respectively increased with the increase in the natural gas injection pressure. Moreover, as for practical engineering application, because of the small density of natural gas, this gas fuel is always stored at a very high pressure. Therefore, high-pressure natural gas injection is necessary for DI engine and plays an important role in the air–fuel mixture and finally the combustion performance.
Methane is the main component of natural gas and has a quite high C–H bond energy, which results in the combustion instability of natural gas. The flame propagation speed of natural gas is lower than that of traditional fossil fuels such as gasoline and diesel, especially under lean-burn conditions.8,9 Thus, natural gas and another fuel are together injected into the engine cylinder as for the dual-fuel combustion. Dual-fuel (for example diesel/NG) compression ignition engine has gained more and more interest due to its good characteristic for clean emissions together with high combustion efficiency.10 In diesel/natural gas dual-fuel engines, the natural gas is the primary fuel to produce power, but natural gas has a relatively low reactivity and is not easy for ignition; thus diesel is adopted as an ignition source to initiate the ignition of the natural gas–air mixture.11 Sevik et al.12 reported that a small pilot injection of diesel (5–10% of the fuel energy) was used to ignite the direct injected natural gas jet in a high-pressure natural gas direct-injection engine. In a dual-fuel (diesel/NG) direct-injection engine, fuels such as both natural gas and diesel should be injected directly into the engine cylinder within a limited time of approximately 2–3 ms per cycle. Within such instantaneous time, the natural gas fuel and the liquid diesel need to mix with air inside the combustion chamber rapidly. Moreover, as the ignition source, diesel needs to evaporate from liquid phase to gaseous phase to get ready for ignition. Therefore, the high-pressure natural gas jet and liquid diesel spray meet each other inside the space-limited combustion chamber, and the interaction between the gaseous and liquid fuels is critical for the air–fuel mixing within limited time and space.
Many studies have been conducted to investigate the fuel injection process and performance of the dual-fuel combustion engines. Daisho et al.13 tested a dual-fuel diesel/natural gas engine by modifying the diesel pilot injection time and found that the advanced diesel injection timing resulted in a better thermal efficiency. Wannatong et al.14 tested a single-cylinder natural gas/diesel dual-fuel engine, and they found that when the pilot diesel quantity was constant while natural gas increased, the peak in-cylinder pressure and the second peak of the heat release rate increased significantly. Liu et al.15 conducted a simulation by coupling the genetic algorithm with a three-dimensional (3D) CFD model of a diesel/natural gas dual-fuel engine and reported that the optimized diesel injection timing is gradually postponed with the increase of the pilot diesel quantity. These above literatures studied the diesel injection in the dual-fuel engine and revealed that the diesel injection causes great effect on the engine performances. Furthermore, the natural gas injection performance is also critical for the mixing process. Distaso et al.16 conducted a 3D simulation of a methane jet in the prechamber of a lean-burning turbulent jet ignition engine, and they demonstrated that the methane jet produced a turbulent flow exiting from the prechamber initiating a scavenging process; this flow had a significant impact on the mixing process in the cylinder. In addition, the high-pressure natural gas injection of the dual-fuel engine has been studied. Ishibashi et al.17 optically observed a natural gas jet and dual-fuel combustion progress in a rapid compression and expansion machine and reported that NG injection parameters, such as the injection time delay and injection angle, had a significant impact on jet entrainment. Felayati et al.10 tested a dual-fuel diesel/natural gas engine and studied the natural gas split injection strategy, and they found that natural gas split injection with proportional split injection ratio and small dwelling timing improved NOx, CO, and hydrocarbons (HC) emissions.
The aforementioned brief discussion revealed that both diesel injection and natural gas jet of the dual-fuel diesel/natural gas engine are important and cause significant influence on the air–fuel mixing. However, those abovementioned studies revealed that the natural gas jet in the engine is not high-pressure injection, while the fuel injection pressure is practically high for the direct-injection mode. Moreover, seldom researches studied the interaction between the high-pressure diesel injection and the high-pressure natural gas jet, which is critical for the mixture formation and the further combustion. Therefore, this work aims at analyzing the interaction between the diesel and the natural gas (here methane) jet under the condition of high pressure. This work conducted the optical experiments of the dual-fuel diesel/methane injection, and both fuel injections were maintained at a high pressure based on a constant-volume vessel (CVV) test rig. The details of the diesel fuel spray and methane jet were observed using a schlieren system together with a digital high-speed camera, and the dual-fuel penetration characteristic was further discussed based on the experimental results.
2. Experimental Investigation
To understand the interaction between the liquid diesel fuel and the natural gas fuel, an optical test rig was designed and built. Further, the methane was adopted as the test gas fuel because methane is the primary component of natural gas in this paper. A constant-volume vessel (CVV) test rig was designed to test both the single-hole diesel and methane injectors under the condition of varied injection pressure and back pressure, as shown in Figure 1. There were two high-pressure injectors for this dual-fuel injection experiment. A BOSCH diesel injector was adopted as the high-pressure diesel injector with the injection pressure up to 160 MPa, and a BOSCH gasoline injector was used to inject methane gas with an injection pressure of up to 30 MPa. To realize the single-hole injection, two matching nozzle covers were designed. For the diesel injector, the cover with a vertical hole had a diameter of 0.26 mm, while for the methane injector, one with a vertical hole had a diameter of 0.3 mm. Both matching covers were respectively connected with the injectors, so that both the diesel and the methane injections became single-hole injections. The single-hole injectors located on the top head of the CVV. The single-hole diesel injector was supplied high-pressure diesel provided by a fuel pump driven by a motor that delivered the fuel into the common rail. A Kistler pressure sensor was mounted to detect the pressures just before the injector. Before the high-pressure fuel from the common rail entered the injector, a regulator was used to adjust the fuel flow to maintain stable flow rate and pressure. At last, the high-pressure diesel fuel was injected into the CVV. The methane that came out from a high-pressure gas tank was boosted by the air pump and finally was delivered to the methane injector. There was an angle φ between the diesel injector central axis and the methane injector central axis, and thus, the methane gas jet could meet the diesel spray.
Figure 1.
Schematics of optical CVB test rig for dual-fuel injection.
In addition, an air pump provided the CVV with compressed air as background gas with back pressure. To observe the spray process of the diesel injection, a schlieren system together with a digital high-speed camera was adopted. Both the injector and the camera were triggered synchronously by an electronically controlled system developed by authors. The images from the camera and the pressure data from the pressure sensor were acquired by a data acquisition system developed by authors, and all data were finally sent to the computer for further data postprocessing. Table 1 presents all the specifications of the test apparatus.
Table 1. Test Apparatus.
| type | specification | ||
|---|---|---|---|
| camera | Photron FASTCAM Mini AX200 | image sensor | CMOS image sensor |
| sensor resolution | 1024 × 1024 pixels | ||
| frame rate | 6400 fps max | ||
| Schlieren system | concave mirror | focal length | 2000 mm ± 10 mm |
| Schlieren diameter | 100 mm | ||
| knife edge | range | 0–10 mm | |
| sensitivity | 0.01 mm | ||
| light source | power | 24 V/300 W | |
| light slit | range | 0–3 mm | |
| sensitivity | 0.01 mm | ||
| pressure sensor | Kistler 4067C3000 | range | 0–300 MPa |
| sensitivity | 5 mV/MPa | ||
| endpoint linearity | <±0.5% FSO | ||
| pressure gauge | R01.4311 | range | 0–10 MPa |
| accuracy | 1% full scale | ||
| operating temperature | –40–60 °C | ||
| fuel pump test bench | Taishang Jinshi 12PSDB | motor power | 15 kW |
| fuel pressure | 0–0.8 MPa | ||
| sensitivity | ±0.01 MPa | ||
| operating speed | 0–1500 rpm | ||
| regulator | NXQ-A(AB)-/20 | maximum mass flow | 10–40 L |
| maximum pressure | 20 MPa | ||
To realize the time-sequence control of dual-fuel injection, we developed an electronically controlled system (ECU) for driving the test rig using an OpenECU Developer Platform M220 and wrote the driving program by adopting Matlab software. This developed ECU can realize flexible adjustment of many control parameters for dual-fuel injections, such as the injection pulse widths of both methane gas and diesel fuel injection, the injection timing of both methane gas and diesel fuel injection, the interval between the methane ignition and the diesel injection and the trigger of the camera.
Furthermore, generally the injection speed of the liquid diesel fuel is much greater than that of the gas methane jet because diesel fuel has greater density and jet inertia. Hence, in this work, the ECU control system was designed to first inject the methane gas and then the liquid diesel fuel injection. The interval of the methane jet and the diesel injection was defined as tinterval, and it was set to 3 ms in this work, i.e., tinterval = 3 ms. Figure 2 shows the sequence of the ECU control program for this dual-fuel injection. ECU synchronously triggered the methane gas injection and the camera. After a short delay tdelay, the liquid diesel fuel was injected into the CVB chamber. The methane gas injection experienced an injection pulse width tCH4, and the diesel fuel injection had an injection pulse width tdiesel. During the optical test, these control parameters were all adjustable through the ECU control system.
Figure 2.
ECU Sequence control of dual-fuel injection.
In this work, the dual-fuel diesel/methane injection tests were conducted under different operation conditions of varied injection pressures together with stable back pressure (the back pressure = 1 MPa). Table 2 is the test conditions.
Table 2. Test Conditions.
| parameters | value | |
|---|---|---|
| methane fuel | injection pressure pCH4 | 15 MPa |
| injection pulse width | 5 ms | |
| injector diameter | 0.3 mm | |
| diesel fuel | diesel type | 0 # diesel in China |
| injection pressure pdiesel | 80, 100, 120 MPa | |
| injection pulse width | 1 ms | |
| single-hole injector diameter | 0.26 mm | |
| background air | air temperature | 300 K |
| air back pressure pb | 1 MPa | |
| injection interval tinterval | 3 ms | |
| angle between two injector central axis φ | 10° degree | |
During the test, first, the CVB was supplied with background air with back pressure pb, and the CVB system was maintained stable for 5 min. In addition, the motor drives the fuel pump to pump the diesel fuel into the rail until the pressure maintains stable. Then, ECU successively began the methane gas fuel injection and the diesel fuel injection, and synchronously, the camera was triggered to record the dual-fuel injection and propagation process. During the experiments, for each operation condition, the test was repeated five times, and the macroparameters of the dual-fuel and diesel injection performances were set to be the average of the five times. The test error was calculated based on the standard deviation method.
Figure 3a gives one optical test image from the camera. It illustrates the gas spray inside the CVB. The images were postprocessed based on the edge extraction method by a program developed in commercial code Matlab by the authors. First, the image prior the injection (i.e., t = 0 ms, no injection) was regarded as the reference image. All other pictures were done the background removing based on the method of grayscale comparison. Figure 3b gives the background removing image. Finally, the postprocessing image was acquired by the threshold segmentation, the edge detection, and the morphology operation, as shown in Figure 3c. It is the postprocessed image, and it clearly shows the dual-fuel jet shape. The dark one is the liquid diesel spray, and the gray one is the methane gas jet. To investigate the details of the dual-fuel injection characteristics, some macroparameters are adopted: the spray tip penetration S, the jet cone area A, and the jet cone perimeter C. There parameters are adopted for both liquid diesel spray and the methane gas jet, as shown in Figure 3c,d. In this work, the spray tip penetration S is defined as the maximum vertical distance of the spray contour profile to the nozzle outlet. Sdiesel and SCH4, respectively, represent the tip penetration of diesel and methane jet.
Figure 3.
Optical image and postprocess image.
For this research, the images of the diesel spray and methane jet are exactly ones in the projection plane. It shows that the diesel spray cone and the methane jet cone overlap with each other, and the area and the perimeter of the gray methane cone are not easy to be deduced due to the overlap. Therefore, we focus on the diesel spray cone and analyze the effect of the high-pressure methane gas jet on the liquid diesel spray. Here, both the jet cone area Adiesel and the jet cone perimeter Cdiesel of diesel are analyzed.
To evaluate the effect of the methane gas jet on the diesel spray, here, we introduce two dimensionless parameters, i.e., the area coefficient a and the perimeter coefficient c. Their definitions are shown in eqs 1 and 2.
 |
1 |
 |
2 |
Moreover, the images show that the diesel spray cone is non-axisymmetric. The vertical central line of the diesel spray cone is defined as y direction. According to the y axis, the cone is divided into two sides, i.e., the left side and the right side. Ag and Aa, respectively, represent the jet area close to the gas jet side and the atmosphere side, while Cg and Ca, respectively, are the jet perimeter close to the gas jet side and the atmosphere side.
To evaluate the dual asymmetry of the diesel spray, the dimensionless parameter of the asymmetry coefficient K is used, and KA and KC, respectively, are the asymmetry area coefficient and the asymmetry perimeter coefficient. Their definitions are shown in eqs 3 and 4.
 |
3 |
 |
4 |
Here, the corner marks g and a, respectively, represent the gas jet side and the atmosphere side of the diesel spray cone.
3. Results and Discussion
Figure 4 shows the images of both the diesel–methane dual-fuel injection and the diesel injection under the same condition of diesel fuel injection = 80 MPa. The methane jet happened first, and then, after 3 ms the diesel injection occurred. The results show that the diesel spray cone (black) is clearer than the methane jet cone due to the greater density difference between diesel fuel and background air. Furthermore, it also reveals that the diesel cone contour has better symmetry for single diesel injection. Once the methane jet occurs in dual-fuel injection, the diesel cone contour changes a lot and has obvious asymmetry, especially the diesel cone front meeting the methane jet cone. The near-nozzle part of the diesel spray cone has few changes because this part does not meet the methane jet. It follows then the methane jet brings perturbation and affects the diesel spray as for the dual-fuel injection condition.
Figure 4.
Images of dual-fuel injection and diesel injection.
Figure 5 presents the penetration characteristics of both the diesel spray and the methane jet of the dual-fuel injection. Figure 5a shows the test results of the tip penetration, and the test deviation is given. Once the methane gas jet first happens, the tip penetration of methane increases. Then, the diesel injection begins at 3 ms with a rapid increasing of the tip penetration. Figure 5b is the penetration velocity deduced by the derivation of the tip penetration in Figure 5a. It reveals that the diesel penetration velocity sharply decreases from the maximum value 230 m/s and then tends to maintain a relative stable value of about 50 m/s. In addition, the methane gas jet has a relative lower penetration speed not more than 40 m/s.
Figure 5.
Penetration characteristics of dual-fuel injection.
Figure 6a gives the tip penetration Sdiesel and the methane tip penetration SCH4 for the dual-fuel injection. It shows that both Sdiesel and SCH4 rise as the dual-fuel injection begins. The methane jet occurs first, and the diesel injection begins at time = 3 ms. Then, the diesel penetrates forward. At time = 3.8 ms, the diesel front covers the methane jet front and soon exceeds the gas jet. Figure 6b gives the penetration velocities of both the diesel and methane jets. It reveals that the diesel penetration sharply decreases from 220 m/s to nearly 50 m/s once it goes into the CVB. The methane jet almost maintains at a relative low speed about 10–30 m/s. The diesel spray obviously has a greater velocity than that of the methane jet.
Figure 6.
Comparison of diesel tip penetration between dual-fuel injection and diesel injection.
Because the diesel spray moves at a greater speed, its spray cone soon overlaps with the methane jet cone. In the images, the methane jet front cannot be seen due to the diesel cover. Therefore, here, we focus on discussing the diesel penetration characteristics in dual-fuel injection. Moreover, in the following discussion, the time zero point is defined as the time when the diesel injection begins.
Figure 6 is the comparison of diesel tip penetration between the dual-fuel injection and the diesel injection in CVB, and it presents the results under the condition of the diesel injection pressure pdiesel = 80 MPa. Figure 7 shows that the diesel tip penetration Sdiesel has an increasing tendency for both the dual-fuel and the diesel injection. There is a crossover point of these two curves at injection time = 0.5 ms. According to this crossover point, the diesel spray penetration process of the dual-fuel injection experiences two stages.
Figure 7.
Comparison of diesel penetration velocity between dual-fuel injection and diesel injection.
Stage I: the early stage (before the turning point), the injection just begins and the fuel locates in the close-to-nozzle zone. During this stage, the diesel tip penetration Sdiesel of the dual-fuel injection is smaller than that of the single diesel injection. This shrinking tip penetration Sdiesel of the dual-fuel injection results in the interference of the methane gas jet. For this dual-fuel injection test, the methane first jets into the CVB with an advanced interval tinterval. After this interval, later the diesel injection occurs, and thus, the diesel spray cone meets the methane jet cone, which compresses the background air and causes a resistance for the diesel penetration. Therefore, the diesel tip penetration decreases due to the methane jet interference.
Stage II: the developing stage (after the turning point), both the diesel and methane continue to penetrate forward in the CVB. Due to the great density of liquid diesel fuel, the diesel spray has greater initial injection momentum than the methane gas jet. As a result, the diesel travels faster than the methane gas jet, and the diesel spray front quickly moves to the methane jet front and exceeds the gas jet. Different from the single diesel injection, the high-pressure methane jet brings turbulent kinetic energy to the static background air, which benefits the breakup of the liquid diesel spray. Many smaller diesel drops occur due to this turbulent interference of the high-pressure methane jet, which may travel faster with smaller mass. In addition, the jet momentum of the methane jet decreases along the jet direction and its front becomes weaker with low pressure. This weak gas jet front causes smaller resistance to the diesel spray. Therefore, the diesel tip penetration Sdiesel of the dual-fuel injection becomes larger than that of the single diesel injection during stage II.
Figure 7 is the diesel penetration velocity Vdiesel under the condition of pinjection = 80 MPa. For both the dual-fuel and the single diesel injections, the diesel penetration velocity decreases sharply in stage I and then tends to slightly drop in stage II. The maximum Vdiesel of the diesel injection is greater than that of the dual-fuel injection in stage I, while Vdiesel of the diesel injection becomes slightly higher than that of the dual-fuel injection in stage II.
Figure 8 gives the diesel spray area Adiesel and Figure 9 is the diesel spray perimeter Cdiesel under the condition of pinjection = 80 MPa. For both the dual-fuel and the single diesel injections, both Adiesel and Cdiesel increase, and these curves also have two-stage characteristic. First, both Adiesel and Cdiesel of diesel injection are higher than those of dual-fuel injection, and then, both Adiesel and Cdiesel of diesel injection become smaller than those of dual-fuel injection. Furthermore, the crossover points of these Adiesel and Cdiesel curves appear later than that of the tip penetration curve; here, it is 0.7 ms. During stage II, many smaller drops appear due to the liquid diesel breakup, and the methane jet improves the breakup of the liquid drops. Thus, the diesel cover area and perimeter increase in stage II.
Figure 8.
Comparison of diesel spray area between dual-fuel injection and diesel injection.
Figure 9.
Comparison of diesel spray perimeter between dual-fuel injection and diesel injection.
Figure 10 gives both the area coefficient a and the perimeter coefficient c under the condition of diesel injection pressure = 80 MPa. Figure 10a shows that the area coefficients a, al, and ar, all sharply rise as the injection early occurs, and they slightly increase later. In stage I (before t < 0.7 ms), a is not more than 1; in stage II (t > 0.7 ms), a becomes larger than 1. Both al and ar have the similar increasing trends as a. However, al is always larger than ar. Similarly, cl is also greater than cr as shown in Figure 10b. This reveals that the left side overlapping with the methane gas jet tends to have larger area and greater perimeter. This shows that during the later stage II, the methane gas jet meets the diesel spray, and the gas jet tends to enhance the diesel atomization of the wide spray cone with a greater area and perimeter. On the contrary, the right side of the diesel spray is relatively far from the methane gas jet, and it tends to have a smaller area and perimeter. The results show that the diesel spray develops toward to the methane gas jet. This is because the high-pressure methane gas jet induces the background gas moving, and this gas flow affects the diesel spray, especially the front cone jet, since there are more breakup drops with a smaller mass in the front part.
Figure 10.
Area coefficient and perimeter coefficient.
This two-stage characteristic of dual-fuel injection also occurs in other conditions of different injection pressures. Figure 11 is the diesel tip penetration under the condition of varied injection pressure. For each dual-fuel injection curve, there is a crossover point marked with a star symbol. It reveals that the crossover point varies as pdiesel changes.
Figure 11.
Diesel tip penetration under the condition of different injection pressures.
Figure 12 shows that effects of the diesel injection pressure on the dual-fuel injection. As the diesel injection pressure rises, the time for dual-fuel meeting becomes shorter. Furthermore, this decreasing tendency occurs also in the time for the crossover point. Both curves have a linear decreasing tendency as the injection pressure pdiesel increases. In addition, both curves have the same slope (here −0.0038). This reveals that the diesel injection pressure directly causes linear influence on the two-stage dual-fuel injection characteristic. As the pdiesel increases, the diesel spray meets the methane jet advancer and the cross point occurs earlier.
Figure 12.
Effect of diesel injection pressure on the dual-fuel injection.
Figure 13 shows the diesel penetration velocity Vdiesel. For all different injection pressure conditions, the curve of diesel penetration velocity Vdiesel sharply drops from a high speed (range in 220–380 m/s) and then tends to maintain as a stable low speed (about 30–50 m/s).
Figure 13.
Diesel spray penetration velocity.
To illustrate clearly the comparison between the two stages, the average penetration velocity of each stage is used for discussion. A dimensionless parameter, i.e., velocity ratio, is defined as the ratio between the average Vdiesel of dual-fuel injection to that of the single diesel injection. Figure 14 gives the velocity ratio of two stages. It shows that the velocity ratio is not more than 1 in stage I, while it is above 1 in stage II. The results demonstrate that the diesel of single injection condition penetrates faster in stage I, while that of the dual-fuel injection moves faster. These results illustrate that the diesel spray is improved due to the methane gas jet, especially in stage II.
Figure 14.
Effects of injection pressure on injection energy.
Figure 15 is the effect of the high diesel injection pressure on the area coefficient a. Figure 15a shows that the area coefficient curve tends to rise under the condition of different injection pressures during the injection process. For injection pressure of both 80 and 100 MPa, first, the area coefficient a is lower than 1 and then increases above 1. Moreover, for higher injection pressure, the time of the turning point tends to be in advance. For a high pressure of 120 MPa, the area coefficient a is always above 1, a > 1 during the whole injection process. This is because as the injection pressure increases, the diesel spray moves faster and meets the methane gas jet earlier. Higher the diesel injection pressure, greater the a. Figure 15b reveals that the area coefficient a nearly linearly increases with the increase in the injection pressure.
Figure 15.
Effect of injection pressure on area coefficient.
The diesel injection pressure has the similar effect on the perimeter coefficient c, as shown in Figure 16. It shows that the perimeter coefficient curve tends to rise under the condition of different injection pressures during the injection process. Similarly, the perimeter coefficient c nearly linearly increases with the increase in the injection pressure.
Figure 16.
Effect of injection pressure on perimeter coefficient.
Figure 17 gives the results of the asymmetry coefficient K. It reveals that most KA and KC are greater than 1 even for different diesel injection pressures. This illustrates that this dual-fuel injection is asymmetric and the methane gas jet enhances this asymmetry so that the spray cone shifts to the side of the methane gas jet. This is because the high-pressure methane gas jet changes the background of the diesel spray, and it increases the resistance for the diesel spray, which may enhance the atomization of the diesel spray. This atomization of the liquid fuel is enhanced especially at the spray cone front close to the gas jet.
Figure 17.
Effect of injection pressure on asymmetry coefficient.
4. Conclusions
This study presents an experimental investigation on the high-pressure dual-fuel diesel/methane injection under conditions of varied pressures, and the dual-fuel injection characteristics are further discussed. The main conclusions are summarized as follows:
The diesel penetration process of the dual-fuel injection experiences two stages. During the early stage I, the diesel tip penetration Sdiesel, the diesel spray area Adiesel, and the diesel spray perimeter Cdiesel of the dual-fuel injection are smaller than those of the single diesel injection. This shrinking tip penetration Sdiesel of the dual-fuel injection results in the interference of the methane gas jet, which in advance compresses the background air and causes a resistance for the diesel penetration. During stage II, both the diesel and methane continue to penetrate forward and Sdiesel, Adiesel, and Cdiesel of the dual-fuel injection become larger than those of the single diesel injection do.
The diesel injection pressure causes an effect on the dual-fuel spray penetration. The diesel injection pressure directly causes linear influence on the two-stage dual-fuel injection characteristic. As the diesel injection pressure increases, the diesel spray meets the methane jet advancer and the cross point occurs linearly earlier.
The dual-fuel injection is asymmetric and the methane gas jet enhances this asymmetry so that the spray cone shifts to the side of the methane gas jet. This is because the high-pressure methane gas jet changes the background of the diesel spray, and it increases the resistance for the diesel spray, which may enhance the atomization of the diesel spray. This atomization of the liquid fuel is enhanced especially at the spray cone front close to the gas jet.
Acknowledgments
The authors gratefully acknowledge the financial support for this work from the National Natural Science Foundation (91641106) and Hunan Natural Science Foundation (2021JJ60056). The authors also acknowledge the Beijing University of Technology for their financial support for this work.
The authors declare no competing financial interest.
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